Electrically conductive polymers are currently the subject to great interest worldwide. Metallic conductors and semiconductors can be replaced by these polymers in a variety of applications, such as batteries, transducers, switches, photocells, circuit boards, heating elements, antistatic protection (ESD), and electromagnetic protection (EMI). The advantages of electrically conductive polymers over metals include their light weight, their mechanical properties, their corrosion resistance, and lower costs of synthesis and cost of processing.
Electrically conductive plastics can roughly be divided into two categories: filled conductive plastics, in which a conductive filler such as carbon black, carbon fiber, metal powder, etc., is added to a thermosetting or thermoplastic resin, and intrinsically conductive plastics, which are based on polymers made conductive by an oxidation or reduction (doping) process.
The electrical conductivity of certain filled conductive plastics is dependent on the inter-particle contacts between the conductive filler particles. In general, approximately 10-50% wt. of a well dispersed filler is required to produce highly conductive plastics materials. However, such conductive plastics materials have certain disadvantages: their mechanical and some chemical properties are substantially degraded as the filler content increases and the polymer content decreases; their electrical conductivity is difficult to control, especially within the semiconductor regime; and stable and homogeneous dispersions of the fillers in the matrix plastic are often difficult to achieve.
Intrinsically conductive polymers can be produced from organic polymers having long chains made up of conjugated double bonds and heteroatoms. The polymers can be rendered electrically conductive by perturbing the .pi.-electron and .pi.-p-electron systems of the conjugated double bonds and heteroatoms by adding to the polymer certain doping agents which will act either as electron acceptors or electron donors. Thereby, electron holes or excess electrons are formed in the polymer chain, enabling electric current to travel along the conjugated chain.
Intrinsically conductive polymers have the advantage in easy modification of the conductivity as a function of the doping time, which is especially evident within low conductivity ranges. By contrast, for certain filled conductive plastics low conductivities are difficult to achieve. Examples of currently known intrinsically conductive polymers include polyacetylene, poly-p-phenylene, polypyrrole, polythiophene and derivatives thereof, and polyaniline and derivatives thereof.
There are two principal methods for processing conductive polymers into desired parts, fibers, films, etc.: melt processing and solution processing. Melt processing is a versatile processing method, whereas solution processing is applicable mainly to the production of fibers and films but not for the production of thick shaped articles. However, the processing and doping of most polymers which are intrinsically conductive involve problems with respect to handling, stability, homogeneity, etc., of the materials.
Polyaniline and derivatives thereof are technically and commercially promising intrinsically conductive polymers. An aniline polymer is composed of aniline units, the nitrogen atom of which is bonded to the carbon in the p-position of the phenyl ring of the next unit. Unsubstituted polyaniline may occur in a plurality of forms, including leucoemeraldine, protoemeraldine, emeraldine, nigraniline, and toluprotoemeraldine.
Sulfonic acid or derivatives thereof are often used as doping agents for polyaniline. Preferably, the polyaniline doping agent is dodecylbenzenesulfonic acid (DBSA), a derivative of sulfonic acid (cf., for example, patent publications WO-9010297 and U.S. Pat. No. 5,002,700).
In order to improve the electrical, optical and mechanical properties of conductive polymer blend/composites and thereby to expand their range of use, efforts have been made to develop blend/composite materials comprising an intrinsically conductive polymer, a doping agent, and a thermoplastic polymer matrix which would give the blend/composite certain required mechanical properties.
In the preparation of polymer blends, efficient blending of the components is essential for the attainment of a satisfactory end result. It is known that one factor having a decisive effect on miscibility is the molar masses of the polymers, i.e. polymers having a lower molar mass blend better with each other.
The effect of the polymerization conditions of chemical polymerization and, for example, the effect of the oxidizing agent to aniline ratio on the properties of polyaniline, such as its electrical conductivity, yield, elemental composition, and degree of oxidation, have been investigated and described in, for example, the following publications: Pron et al., Synthetic Metals 1988, Vol. 24, p. 193; Armes et al., Synthetic Metals 1988, Vol. 22, p. 283; Asturias et al., Synthetic Metals 1989, Vol. 29, E157. The effect of the conditions of chemical polymerization on the inherent viscosity of polyaniline, which correlates with the molecular weight of the polymer, have been investigated and explained by Yong Cao et al. in Polymer, 1989, 30, Dec. This article discusses, among other things, the effects of oxidizing agents and protonic acids, the aniline to oxidizing agent molar ratio, pH, polymerization temperature and polymerization time, particularly with respect to the viscosity and conductivity of the polymer. The polymerization of aniline according to the methods of this article was carried out in a conventional manner either by adding a solution of HCl and oxidizing agent (the oxidizing agent being, for example, (NH.sub.4).sub.2 S.sub.2 O.sub.8, K.sub.2 Cr.sub.2 O.sub.7, KIO.sub.3, FeCl.sub.3. KMnO.sub.4, KBRO.sub.3, KClO.sub.3 and H.sub.2 O.sub.2 into a reaction vessel which containing aniline, or by adding the oxidizing agent into a reaction vessel which contained a solution of aniline and HCl. By varying the parameters mentioned above, polyanilines of different yields, viscosities and conductivities were obtained.